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Slavery had mostly died out in western Europe about the year 1000, replaced by serfdom. It lingered longer in England and in peripheral areas linked to the Muslim world, where slavery continued to flourish. Church rules suppressed slavery of Christians. Most historians argue the transition was quite abrupt around 1000, but some see a gradual transition from about 300 to 1000.
The chaos following the barbarian invasions of the Roman Empire made the taking of slaves habitual throughout Europe in the early Middle Ages. Roman practices continued in many areas – the Welsh laws of Hywel the Good included provisions dealing with slaves – and Germanic laws provided for the enslavement of criminals, as when the Visigothic Code prescribed enslavement for those who could not pay the financial penalty for their crime and as a punishment for certain other crimes. Such criminals would become slaves to their victims, often with their property.
As these peoples Christianized, the church worked more actively to reduce the practice of holding coreligionists in bondage. St. Patrick, who himself was captured and enslaved at one time, protested an attack that enslaved newly baptized Christians in his letter to the soldiers of Coroticus. The restoration of order and the growing power of the church slowly transmuted the late Roman slave system of Diocletian into serfdom.
Another major factor was the rise of Bathilde, queen of the Franks, who had been enslaved before marrying Clovis II. When she became regent, her government outlawed slave-trading of Christians throughout the Merovingian empire.
About 10% of England's population entered in the Domesday Book (1086) were slaves, despite chattel slavery of English Christians being nominally discontinued after the 1066 conquest. It is difficult to be certain about slave numbers, however, since the old Roman word for slave (servus) continued to be applied to people with a status that was later to be called "serf."
Demand from the Islamic world dominated the slave trade in medieval Europe. For most of that time, however, sale of Christian slaves to non-Christians was banned. In the pactum Lotharii of 840 between Venice and the Carolingian Empire, Venice promised not to buy Christian slaves in the Empire, and not to sell Christian slaves to Muslims. The Church prohibited the export of Christian slaves to non-Christian lands, for example in the Council of Koblenz in 922, the Council of London in 1102, and the Council of Armagh in 1171.
As a result, most Christian slave merchants focused on moving slaves from non-Christian areas to Muslim Spain, North Africa, and the Middle East, and most non-Christian merchants, although not bound by the Church's rules, focused on Muslim markets as well. Arabic silver dirhams, presumably exchanged for slaves, are plentiful in eastern Europe and Southern Sweden, indicating trade routes from Slavic to Muslim territory.
By the reign of Pope Zacharias (741–752), Venice had established a thriving slave trade, buying in Italy, amongst other places, and selling to the Moors in Northern Africa (Zacharias himself reportedly forbade such traffic out of Rome). When the sale of Christians to Muslims was banned (pactum Lotharii ), the Venetians began to sell Slavs and other Eastern European non-Christian slaves in greater numbers. Caravans of slaves traveled from Eastern Europe, through Alpine passes in Austria, to reach Venice. A record of tolls paid in Raffelstetten (903–906), near St. Florian on the Danube, describes such merchants. Some are Slavic themselves, from Bohemia and the Kievan Rus'. They had come from Kiev through Przemyśl, Kraków, Prague, and Bohemia. The same record values female slaves at a tremissa (about 1.5 grams of gold or roughly 1/3 of a Dinar) and male slaves, who were more numerous, at a saiga (which is much less). Eunuchs were especially valuable, and "castration houses" arose in Venice, as well as other prominent slave markets, to meet this demand.
Venice was far from the only slave trading hub in Italy. Southern Italy boasted slaves from distant regions, including Lombardy, Greece, Bulgaria, Armenia, and Slavic regions. During the 9th and 10th centuries, Amalfi was a major exporter of slaves to North Africa. Genoa, along with Venice, dominated the trade in the Eastern Mediterranean beginning in the 12th century, and in the Black Sea beginning in the 13th century. They sold both Slavic and Baltic slaves, as well as Georgians, Turks, and other ethnic groups of the Black Sea and Caucasus, to the Muslim nations of the Middle East. Genoa primarily managed the slave trade from Crimea to Mamluk Egypt, until the 13th century, when increasing Venetian control over the Eastern Mediterranean allowed Venice to dominate that market. Between 1414 and 1423 alone, at least 10,000 slaves were sold in Venice.
Records of long-distance Jewish slave merchants date at least as far back as 492, when Pope Gelasius permitted Jews to import non-Christian slaves into Italy, at the request of a Jewish friend from Telesina.   By the turn of the 6th to the 7th century, Jews had become the chief slave traders in Italy, and were active in Gaelic territories. Pope Gregory the Great issued a ban on Jews possessing Christian slaves, lest the slaves convert to Judaism.  By the 9th and 10th centuries, Jewish merchants, sometimes called Radhanites, were a major force in the slave trade continent-wide.  
Jews were one of the few groups who could move and trade between the Christian and Islamic worlds. Ibn Khordadbeh observed and recorded routes of Jewish merchants in his Book of Roads and Kingdoms from the South of France to Spain, carrying (amongst other things) female slaves, eunuch slaves, and young slave boys. He also notes Jews purchasing Slavic slaves in Prague.   Letters of Agobard, archbishop of Lyons (816-840),    acts of the emperor Louis the Pious,  and the seventy-fifth canon of the Council of Meaux of 845 confirms the existence of a route used by Jewish traders with Slavic slaves through the Alps to Lyon, to Southern France, to Spain. Toll records from Walenstadt in 842–843 indicate another trade route, through Switzerland, the Septimer and Splügen passes, to Venice, and from there to North Africa.
As German rulers of Saxon dynasties took over the enslavement (and slave trade) of Slavs in the 10th century, Jewish merchants bought slaves at the Elbe, sending caravans into the valley of the Rhine. Many of these slaves were taken to Verdun, which had close trade relations with Spain. Many would be castrated and sold as eunuchs as well. 
Jews would later become highly influential in the European slave trade, reaching their apex from the 16th to 19th centuries.
Muslim Spain imported an enormous number[clarification needed] of slaves, as well as serving as a staging point for Muslim and Jewish merchants to market slaves to the rest of the Islamic world. During the reign of Abd-ar-Rahman III (912–961), there were at first 3,750, then 6,087, and finally 13,750 Saqaliba, or Slavic slaves, at Córdoba, capital of the Umayyad Caliphate. Ibn Hawqal, Ibrahim al-Qarawi, and Bishop Liutprand of Cremona note that the Jewish merchants of Verdun specialized in castrating slaves, to be sold as eunuch saqaliba, which were enormously popular[clarification needed] in Muslim Spain.  
During the Viking age (793 – approximately 1100), the Norse raiders often captured and enslaved militarily weaker peoples they encountered. The Nordic countries called their slaves thralls (Old Norse: Þræll). The thralls were mostly from Western Europe, among them many Franks, Anglo-Saxons, and Celts. Many Irish slaves travelled in expeditions for the colonization of Iceland. Raids on monasteries provided a source of young, educated slaves who could be sold in Venice or Byzantium for high prices. Scandinavian trade centers stretched eastwards from Hedeby in Denmark and Birka in Sweden to Staraya Ladoga in northern Russia before the end of the 8th century.
This traffic continued into the 9th century as Scandinavians founded more trade centers at Kaupang in southwestern Norway and Novgorod, farther south than Staraya Ladoga, and Kiev, farther south still and closer to Byzantium. Dublin and other northwestern European Viking settlements were established as gateways through which captives were traded northwards. In the Laxdæla saga, for example, a Rus merchant attends a fair in the Brenn Isles in Sweden selling female slaves from northwestern Europe.
The Norse also took German, Baltic, Slavic and Latin slaves. The 10th-century Persian traveller Ibn Rustah described how Swedish Vikings, the Varangians or Rus, terrorized and enslaved the Slavs taken in their raids along the Volga River. Slaves were often sold south, to Byzantine or Muslim buyers, via paths such as the Volga trade route. Ahmad ibn Fadlan of Baghdad provides an account of the other end of this trade route, namely of Volga Vikings selling Slavic Slaves to middle-eastern merchants. Finland proved another source for Viking slave raids. Slaves from Finland or Baltic states were traded as far as central Asia. 
The Mongol invasions and conquests in the 13th century added a new force in the slave trade. The Mongols enslaved skilled individuals, women and children and marched them to Karakorum or Sarai, whence they were sold throughout Eurasia. Many of these slaves were shipped to the slave market in Novgorod.  
Genoese and Venetians merchants in Crimea were involved in the slave trade with the Golden Horde.  In 1441, Haci I Giray declared independence from the Golden Horde and established the Crimean Khanate. For a long time, until the early 18th century, the khanate maintained a massive[clarification needed] slave trade with the Ottoman Empire and the Middle East. In a process called the "harvesting of the steppe", they enslaved many Slavic peasants.
As a commonly traded commodity in the British Isles, like cattle, slaves could become a form of internal or trans-border currency. William the Conqueror banned the exporting of slaves from England, limiting the nation's participation in the slave trade.
Although the primary flow of slaves was toward Muslim countries,[further explanation needed] Christians did acquire Muslim slaves; in Southern France, in the 13th century, "the enslavement of Muslim captives was still fairly common". There are records, for example, of Saracen slave girls sold in Marseilles in 1248, a date which coincided with the fall of Seville and its surrounding area, to raiding Christian crusaders, an event during which a large number of Muslim women from this area, were enslaved as war booty, as it has been recorded in some Arabic poetry, notably by the poet al-Rundi, who was contemporary to the events.
Christians also sold Muslim slaves captured in war. The Order of the Knights of Malta attacked pirates and Muslim shipping, and their base became a center for slave trading, selling captured North Africans and Turks. Malta remained a slave market until well into the late 18th century. One thousand slaves were required to man the galleys (ships) of the Order. 
As more and more of Europe Christianized, and open hostilities between Christian and Muslim nations intensified, large-scale slave trade moved to more distant sources. Sending slaves to Egypt, for example, was forbidden by the papacy in 1317, 1323, 1329, 1338, and, finally, 1425, as slaves sent to Egypt would often become soldiers, and end up fighting their former Christian owners. Although the repeated bans indicate that such trade still occurred, they also indicate that it became less desirable. In the 16th century, African slaves replaced almost all other ethnicities and religious enslaved groups in Europe.
Slavery was heavily regulated in Roman law, which was reorganized in the Byzantine Empire by Justinian I as the Corpus Iuris Civilis. Although the Corpus was lost to the West for centuries, it was rediscovered in the 11th and 12th centuries, and led to the foundation of law schools in Italy and France. According to the Corpus, the natural state of humanity is freedom, but the "law of nations" may supersede natural law and reduce certain people to slavery. The basic definition of slave in Romano-Byzantine law was:
It was, however, possible to become a freedman or a full citizen; the Corpus, like Roman law, had extensive and complicated rules for manumission of slaves.
The slave trade in England was officially abolished in 1102.
Medieval canon lawyers concluded that slavery was contrary to the spirit of Christianity, and by the 11th century when almost all of Europe had been Christianized, the laws of slavery in civil law codes were now antiquated and unenforceable. There were a number of areas where Christians lived with non-Christians, such as Al-Andalus and Sicily, the crusader states, and in the still-pagan areas of northeastern Europe; therefore, canon law permitted Christians to keep non-Christian slaves, as long as these slaves were treated humanely and were freed if they chose to convert to Christianity. In fact, there was an explicit legal justification for the enslavement of Muslims, found in the Decretum Gratiani and later expanded upon by the 14th century jurist Oldradus de Ponte: the Bible states that Hagar, the slave girl of Abraham, was beaten and cast out by Abraham's wife Sarah. A popular medieval legend held that Muslims were the descendents of Hagar, while Christians descended from the legitimate marriage of Abraham and Sarah. By extension it was therefore permitted for Christians to enslave Muslims.
The Decretum, like the Corpus, defined a slave as anyone whose mother was a slave. Otherwise, the canons were concerned with slavery only in ecclesiastical contexts: slaves were not permitted to marry or to be ordained as clergy.
In the crusader Kingdom of Jerusalem, founded in 1099, at most 120,000 Franks ruled over 350,000 Muslims, Jews, and native Eastern Christians. Following the initial invasion and conquest, sometimes accompanied by massacres or expulsions of Jews and Muslims, a peaceable co-existence between followers of the three religions prevailed. The Crusader states inherited many slaves. To this may have been added some Muslims taken as captives of war. The Kingdom's largest city, Acre, had a large slave market; however, the vast majority of Muslims and Jews remained free. The laws of Jerusalem declared that former Muslim slaves, if genuine converts to Christianity, must be freed.
In 1120, the Council of Nablus forbade sexual relations between crusaders and their female slaves: if a man raped his own slave, he would be castrated, but if he raped someone else's slave, he would be castrated and exiled from the kingdom. But Benjamin Z. Kedar argued that the canons of the Council of Nablus were in force in the 12th century but had fallen out of use by the thirteenth. Marwan Nader questions this and suggests that the canons may not have applied to the whole kingdom at all times.
No Christian, whether Western or Eastern, was permitted by law to be sold into slavery, but this fate was as common for Muslim prisoners of war as it was for Christian prisoners taken by the Muslims.
The 13th-century Assizes of Jerusalem dealt more with fugitive slaves and the punishments ascribed to them, the prohibition of slaves testifying in court, and manumission of slaves, which could be accomplished, for example, through a will, or by conversion to Christianity. Conversion was apparently used as an excuse to escape slavery by Muslims who would then continue to practise Islam; crusader lords often refused to allow them to convert, and Pope Gregory IX, contrary to both the laws of Jerusalem and the canon laws that he himself was partially responsible for compiling, allowed for Muslim slaves to remain enslaved even if they had converted.
The medieval Iberian Peninsula was the scene of almost constant warfare among Muslims and Christians (though not always aligned by religion). Periodic raiding expeditions were sent from Al-Andalus to ravage the Christian Iberian kingdoms, bringing back booty and people. For example, in a raid on Lisbon in 1189 the Almohad caliph Yaqub al-Mansur took 3,000 female and child captives, and his governor of Córdoba took 3,000 Christian slaves in a subsequent attack upon Silves in 1191; an offensive by Alfonso VIII of Castile in 1182 brought him over two-thousand Muslim slaves.
Contrary to suppositions of historians such as Marc Bloch, slavery thrived as an institution in medieval Christian Iberia. Slavery existed in the region under the Romans, and continued to do so under the Visigoths. From the fifth to the early 8th century, large portions of the Iberian Peninsula were ruled by Christian Visigothic Kingdoms, whose rulers worked to codify human bondage. In the 7th century, King Chindasuinth issued the Visigothic Code (Liber Iudiciorum), to which subsequent Visigothic kings added new legislation. Although the Visigothic Kingdom collapsed in the early 8th century, portions of the Visigothic Code were still observed in parts of Spain in the following centuries. The Code, with its pronounced and frequent attention to the legal status of slaves, reveals the continuation of slavery as an institution in post-Roman Spain.
The Code regulated the social conditions, behavior, and punishments of slaves in early medieval Spain. The marriage of slaves and free or freed people was prohibited. Book III, title II, iii ("Where a Freeborn Woman Marries the Slave of Another or a Freeborn Man the Female Slave of Another") stipulates that if a free woman marries another person's slave, the couple is to be separated and given 100 lashes. Furthermore, if the woman refuses to leave the slave, then she becomes the property of the slave's master. Likewise, any children born to the couple would follow the father's condition and be slaves.
Unlike Roman law, in which only slaves were liable to corporal punishment, under Visigothic law, people of any social status were subject to corporal punishment. However, the physical punishment, typically beatings, administered to slaves was consistently harsher than that administered to freed or free people. Slaves could also be compelled to give testimony under torture. For example, slaves could be tortured to reveal the adultery of their masters, and it was illegal to free a slave for fear of what he or she might reveal under torture. Slaves' greater liability to physical punishment and judicial torture suggests their inferior social status in the eyes of Visigothic lawmakers.
Slavery remained persistent in Christian Iberia after the Umayyad invasions in the 8th century, and the Visigothic law codes continued to control slave ownership. However, as William Phillips notes, medieval Iberia should not be thought of as a slave society, but rather as a society that owned slaves. Slaves accounted for a relatively small percentage of the population, and did not make up a significant portion of the labor pool. Furthermore, while the existence of slavery continued from the earlier period, the use of slaves in post-Visigothic Christian Iberia differed from early periods. Ian Wood has suggests that, under the Visigoths, the majority of the slave population lived and worked on rural estates.
After the Muslim invasions, slave owners (especially in the kingdoms of Aragon and Valencia) moved away from using slaves as field laborers or in work gangs, and did not press slaves into military service. Slaves tended to be owned singly rather than in large groups. There appear to have been many more female than male slaves, and they were most often used as domestic servants, or to supplement free labor. In this respect, slave institutions in Aragon, especially, closely resembled those of other Mediterranean Christian kingdoms in France and Italy.
In the kingdoms of León and Castile, slavery followed the Visigothic model more closely than in the littoral kingdoms. Slaves in León and Castile were more likely to be employed as field laborers, supplanting free labor to support an aristocratic estate society. These trends in slave populations and use changed in the wake of the Black Death in 1348, which significantly increased the need for slaves across the whole of the peninsula.
Christians were not the only slaveholders in Christian Iberia. Both Jews and Muslims living under Christian rule owned slaves, though more commonly in Aragon and Valencia than in Castile. After the conquest of Valencia in 1245, the Kingdom of Aragon prohibited the possession of Christian slaves by Jews, though they were still permitted to hold Muslim or pagan slaves. The main role of Iberian Jews in the slave trade came as facilitators: Jews acted as slave brokers and agents of transfer between the Christian and Muslim kingdoms.
This role caused some degree of fear among Christian populations. A letter from Pope Gregory XI to the Bishop of Cordoba in 1239 addressed rumors that the Jews were involved in kidnapping and selling Christian women and children into slavery while their husbands were away fighting the Muslims. Despite these worries, the primary role of Jewish slave traders lay in facilitating the exchange of captives between Muslim and Christian rulers, one of the primary threads of economic and political connectivity between Christian and Muslim Iberia.<
In the early period after the fall of the Visigothic kingdom in the 8th century, slaves primarily came into Christian Iberia through trade with the Muslim kingdoms of the south. Most were Eastern European, captured in battles and raids, with the heavy majority being Slavs. However, the ethnic composition of slaves in Christian Iberia shifted over the course of the Middle Ages. Slaveholders in the Christian kingdoms gradually moved away from owning Christians, in accordance with Church proscriptions. In the middle of the medieval period most slaves in Christian Iberia were Muslim, either captured in battle with the Islamic states from the southern part of the peninsula, or taken from the eastern Mediterranean and imported into Iberia by merchants from cities such as Genoa.
The Christian kingdoms of Iberia frequently traded their Muslim captives back across the border for payments of money or kind. Indeed, historian James Broadman writes that this type of redemption offered the best chance for captives and slaves to regain their freedom. The sale of Muslim captives, either back to the Islamic southern states or to third-party slave brokers, supplied one of the means by which Aragon and Castile financed the Reconquista. Battles and sieges provided large numbers of captives; after the siege of Almeria in 1147, sources report that Alfonso VII of León sent almost 10,000 of the city's Muslim women and children to Genoa to be sold into slavery as partial repayment of Genoese assistance in the campaign.
Towards the end of the Reconquista, however, this source of slaves became increasingly exhausted. Muslim rulers were increasingly unable to pay ransoms, and the Christian capture of large centers of population in the south made wholesale enslavement of Muslim populations impractical. The loss of an Iberian Muslim source of slaves further encouraged Christians to look to other sources of manpower. Beginning with the first Portuguese slave raid in sub-Saharan Africa in 1411, the focus of slave importation began to shift from the Mediterranean to the Atlantic World, and the racial composition of slaves in Christian Iberia began to include an increasing number of black Africans.
Between 1489 and 1497 almost 2,100 black slaves were shipped from Portugal to Valencia. By the end of the 15th century, Spain held the largest population of black Africans in Europe, with a small, but growing community of black ex-slaves. In the mid 16th century Spain imported up to 2,000 black African slaves annually through Portugal, and by 1565 most of Seville's 6,327 slaves (out of a total population of 85,538) were black Africans.
Slavery (Romanian: robie, from Slavic rob, slave) existed on the territory of present-day Romania from before the founding of the principalities of Moldavia and Wallachia in the 13th–14th centuries, until it was abolished in stages during the 1840s and 1850s. Most of the slaves were of Roma (Gypsy) ethnicity. Particularly in Moldavia there were also slaves of Tatar ethnicity, probably prisoners captured from the wars with the Nogai and Crimean Tatars.
The exact origins of slavery in the Danubian Principalities are not known. There is some debate over whether the Romani people came to Wallachia and Moldavia as free men or as slaves. In the Byzantine Empire, they were slaves of the state and it seems the situation was the same in Bulgaria and Serbia until their social organization was destroyed by the Ottoman conquest, which would suggest that they came as slaves who had a change of 'ownership'.
Historian Nicolae Iorga associated the Roma people's arrival with the 1241 Mongol invasion of Europe and considered their slavery as a vestige of that era, the Romanians taking the Roma from the Mongols as slaves and preserving their status. Other historians consider that they were enslaved while captured during the battles with the Tatars. The practice of enslaving prisoners may also have been taken from the Mongols. The ethnic identity of the "Tatar slaves" is unknown, they could have been captured Tatars of the Golden Horde, Cumans, or the slaves of Tatars and Cumans.
While it is possible that some Romani people were slaves or auxiliary troops of the Mongols or Tatars, the bulk of them came from south of the Danube at the end of the 14th century, some time after the foundation of Wallachia. By then, the institution of slavery was already established in Moldavia and possibly in both principalities, but the arrival of the Roma made slavery a widespread practice. The Tatar slaves, smaller in numbers, were eventually merged into the Roma population.
The ancient and medieval Near East includes modern day Turkey, the Levant and Egypt, with strong connections to the rest of the north African coastline. All of these areas were ruled by either the Byzantines or the Persians at the beginning of late antiquity. Pre-existing Byzantine (i.e. Roman) and Persian institutions of slavery may have influenced the development of institutions of slavery in Islamic law and jurisprudence.  Likewise, some scholars have argued for the influence of Rabbinic tradition on the development of Islamic legal thought. 
Whatever the relationship between these different legal traditions, many similarities exist between the practice of Islamic slavery in the early Middle Ages and the practices of early medieval Byzantines and western Europeans. The status of freed slaves under Islamic rule, who continued to owe services to their former masters, bears a strong similarity to ancient Roman and Greek institutions. However, the practice of slavery in the early medieval Near East also grew out of slavery practices in currency among pre-Islamic Arabs.
Like the Old and New Testaments and Greek and Roman law codes, the Quran takes the institution of slavery for granted, though it urges kindness toward slaves and eventual manumission, especially for slaves who convert to Islam.  In early Middle Ages, many slaves in Islamic society served as such for only a short period of time—perhaps an average of seven years.  Like their European counterparts, early medieval Islamic slave traders preferred slaves who were not co-religionists and hence focused on "pagans" from inner Asia, Europe, and especially from sub-Saharan Africa.  The practice of manumission, may have contributed to the integration of former slaves into the wider society. However, under sharia law, conversion to Islam did not necessitate manumission. 
Slaves were employed in heavy labor as well as in domestic contexts. Because of Quranic sanction of concubinage,  early Islamic traders, in contrast to Byzantine and early modern slave traders, imported large numbers of female slaves.  The very earliest Islamic states did not create corps of slave soldiers (a practice familiar from later contexts) but did integrate freedmen into armies, which may have contributed to the rapid expansion of early Islamic conquest.  By the 9th century, use of slaves in Islamic armies, particularly Turks in cavalry units and Africans in infantry units, was a relatively common practice. 
In Egypt, Ahmad ibn Tulun imported thousands of black slaves to wrestle independence from the Abbasid Caliphate in Iraq in 868.  The Ikhshidid dynasty used black slave units to liberate itself from Abbasid rule after the Abbasids destroyed ibn Tulun's autonomous empire in 935.  Black professional soldiers were most associated with the Fatimid dynasty, which incorporated more professional black soldiers than the previous two dynasties.  It was the Fatimids who first incorporated black professional slave soldiers into the cavalry, despite massive opposition from Central Asian Turkish Mamluks, who saw the African contingent as a threat to their role as the leading military unit in the Egyptian army. 
In the later half of the Middle Ages, the expansion of Islamic rule further into the Mediterranean, the Persian Gulf, and Arabian Peninsula established the Saharan-Indian Ocean slave trade.  This network was a large market for African slaves, transporting approximately four million African slaves from its 7th century inception to its 20th century demise.  Ironically, the consolidation of borders in the Islamic Near East changed the face of the slave trade.  A rigid Islamic code, coupled with crystallizing frontiers, favored slave purchase and tribute over capture as lucrative slave avenues.  Even the sources of slaves shifted from the Fertile Crescent and Central Asia to Indochina and the Byzantine Empire. 
Patterns of preference for slaves in the Near East, as well as patterns of use, continued into the later Middle Ages with only slight changes. Slaves were employed in many activities, including agriculture, industry, the military, and domestic labor. Women were prioritized over men, and usually served in the domestic sphere as menials, concubines, or wives.  Domestic and commercial slaves were mostly better off than their agricultural counterparts, either becoming family members or business partners rather than condemned to a grueling life in a chain gang. There are references to gangs of slaves, mostly African, put to work in drainage projects in Iraq, salt and gold mines in the Sahara, and sugar and cotton plantations in North Africa and Spain. References to this latter type of slavery are rare, however.  Eunuchs, interestingly, were the most prized and sought-after type of slave.
The most fortunate slaves found employment in politics or the military. In the Ottoman Empire, the Devşrime system groomed young slave boys for civil or military service.  Young Christian boys were uprooted from their conquered villages periodically as a levy, and were employed in government, entertainment, or the army, depending on their talents.  Slaves attained great success from this program, some winning the post of Grand Vizier to the Sultan and others positions in the Janissaries. 
It is a bit of a misnomer to classify these men as "slaves," because in the Ottoman Empire, they were referred to as kul, or, slaves "of the Gate," or Sultanate.  While not slaves per se under Islamic Law, these Devşrime alumni remained under the Sultan's discretion.
The Islamic Near East extensively relied upon professional slave soldiers, and was known for having them compose the core of armies.  The institution was conceived out of political predicaments and reflected the attitudes of the time, and was not indicative of political decline or financial bankruptcy.  Slave units were desired because of their unadulterated loyalty to the ruler, since they were imported and therefore couldn’t threaten the throne with local loyalties or alliances.
Slavery was an important part of Ottoman society. The Byzantine-Ottoman wars and the Ottoman wars in Europe brought large numbers of Christian slaves into the Ottoman Empire. In the middle of the 14th century, Murad I built his own personal slave army called the Kapıkulu. The new force was based on the sultan's right to a fifth of the war booty, which he interpreted to include captives taken in battle. The captive slaves were converted to Islam and trained in the sultan's personal service.
In the devşirme (translated "blood tax" or "child collection"), young Christian boys from Anatolia and the Balkans were taken away from their homes and families, converted to Islam and enlisted into special soldier classes of the Ottoman army. These soldier classes were named Janissaries, the most famous branch of the Kapıkulu. The Janissaries eventually became a decisive factor in the Ottoman military conquests in Europe.
Most of the military commanders of the Ottoman forces, imperial administrators and de facto rulers of the Ottoman Empire, such as Pargalı İbrahim Pasha and Sokollu Mehmet Paşa, were recruited in this way. By 1609 the Sultan's Kapıkulu forces increased to about 100,000.
The concubines of the Ottoman Sultan consisted chiefly of purchased slaves. Because Islamic law forbade Muslims to enslave fellow Muslims, the Sultan's concubines were generally of Christian origin. The mother of a Sultan, though technically a slave, received the extremely powerful title of Valide Sultan, and at times became effective ruler of the Empire (see Sultanate of women). One notable example was Kösem Sultan, daughter of a Greek Christian priest, who dominated the Ottoman Empire during the early decades of the 17th century. Another notable example was Roxelana, the favourite wife of Suleiman the Magnificent.
In Kievan Rus and Russia, the slaves were usually classified as kholops. A kholop's master had unlimited power over his life: he could kill him, sell him, or use him as payment upon a debt. The master, however, was responsible before the law for his kholop's actions. A person could become a kholop as a result of capture, selling himself or herself, being sold for debts or committed crimes, or marriage to a kholop. Until the late 10th century, the kholops represented a majority among the servants who worked lordly lands.
By the 16th century, slavery in Russia consisted mostly of those who sold themselves into slavery owing to poverty. They worked predominantly as household servants, among the richest families, and indeed generally produced less than they consumed. Laws forbade the freeing of slaves in times of famine, to avoid feeding them, and slaves generally remained with the family a long time; the Domostroy, an advice book, speaks of the need to choose slaves of good character and to provide for them properly. Slavery remained a major institution in Russia until 1723, when Peter the Great converted the household slaves into house serfs. Russian agricultural slaves were formally converted into serfs earlier in 1679.
In 1382 the Golden Horde under Khan Tokhtamysh sacked Moscow, burning the city and carrying off thousands of inhabitants as slaves. For years the Khanates of Kazan and Astrakhan routinely made raids on Russian principalities for slaves and to plunder towns. Russian chronicles record about 40 raids of Kazan Khans on the Russian territories in the first half of the 16th century. In 1521, the combined forces of Crimean Khan Mehmed I Giray and his Kazan allies attacked Moscow and captured thousands of slaves. About 30 major Tatar raids were recorded into Muscovite territories between 1558 and 1596. In 1571, the Crimean Tatars attacked and sacked Moscow, burning everything but the Kremlin and taking thousands of captives as slaves. In Crimea, about 75% of the population consisted of slaves.
The laws from 12th and 13th centuries describe the legal status of two categories. According to the Norwegian Gulating code (in about 1160), domestic slaves could not, unlike foreign slaves, be sold out of the country. This and other laws defined slaves as their master's property at the same level as cattle. It also described a procedure for giving a slave their freedom. A freed slave did not have full legal status; for example, the punishment for killing a former slave was low. A former slave's son also had a lower status, but higher than that of his parents. The Norwegian law code from 1274, Landslov (Land's law), does not mention slaves, but former slaves. Thus it seems like slavery was abolished in Norway by this time. In Sweden, slavery was abolished in 1343.
British Wales and Gaelic Ireland and Scotland were among the last areas of Christian Europe to give up their institution of slavery. Under Gaelic custom, prisoners of war were routinely taken as slaves. During the period that slavery was disappearing across most of western Europe, it was reaching its height in the British Isles: the Viking invasions and the subsequent warring between Scandinavians and the natives, the number of captives taken as slaves drastically increased. The Irish church was vehemently opposed to slavery and blamed the 1169 Norman invasion on divine punishment for the practice, along with local acceptance of polygyny and divorce.
In considering how serfdom evolved from slavery, historians who study the divide between slavery and serfdom encounter several issues of historiography and methodology. Some historians believe that slavery transitioned into serfdom (a view that has only been around for the last 200 years), though there is disagreement among them regarding how rapid this transition was. Pierre Bonnassie, a medieval historian, thought that the chattel slavery of the ancient world ceased to exist in the Europe of the 10th century and was followed by feudal serfdom. Jean-Pierre Devroey thinks that the shift from slavery to serfdom was gradual as well in some parts of the continent. Other areas, though, did not have what he calls "western-style serfdom" after the end of slavery, such as the rural areas of the Byzantine Empire, Iceland, and Scandinavia. Complicating this issue is that regions in Europe often had both serfs and slaves simultaneously.
Generally speaking, regarding how slaves differed from serfs, the underpinnings of slavery and serfdom are debated as well. Dominique Barthélemy, among others, has questioned the very premises for neatly distinguishing serfdom from slavery, arguing that a binary classification masks the many shades of servitude. Of particular interest to historians is the role of serfdom and slavery within the state, and the implications that held for both serf and slave. Some think that slavery was the exclusion of people from the public sphere and its institutions, whereas serfdom was a complex form of dependency that usually lacked a codified basis in the legal system. Wendy Davies argues that serfs, like slaves, also became excluded from the public judicial system and that judicial matters were attended to in the private courts of their respective lords.
Despite the scholarly disagreement, it is possible to piece together a general picture of slavery and serfdom. Slaves typically owned no property, and were in fact the property of their masters. Slaves worked full-time for their masters and operated under a negative incentive structure; in other words, failure to work resulted in physical punishment. Serfs held plots of land, which was essentially a form of "payment" that the lord offered in exchange for the serf's service. Serfs worked part-time for the masters and part-time for themselves and had opportunities to accumulate personal wealth that often did not exist for the slave.
Slaves were generally imported from foreign countries or continents, brought to Europe via the slave trade. Serfs were typically indigenous Europeans and were not subject to the same involuntary movements as slaves. Serfs worked in family units, whereas the concept of family was generally murkier for slaves. At any given moment, a slave's family could be torn apart via trade, and masters often used this threat to coerce compliant behavior from the slave.
The end of serfdom is also debated, with Georges Duby pointing to the early 12th century as a rough end point for "serfdom in the strict sense of the term". Other historians dispute this assertion, citing discussions and the mention of serfdom as an institution during later dates (such as in 13th century England, or in Central Europe, where the rise of serfdom coincided with its decline in Western Europe). There are several approaches to get a time span for the transition, and lexicography is one such method. There is supposedly a clear shift in diction when referencing those who were either slaves or serfs at approximately 1000, though there is not a consensus on how significant this shift is, or if it even exists.
In addition, numismatists shed light on the decline of serfdom. There is a widespread theory that the introduction of currency hastened the decline of serfdom because it was preferable to pay for labor rather than depend on feudal obligations. Some historians argue that landlords began selling serfs their land – and hence, their freedom – during periods of economic inflation across Europe. Other historians argue that the end of slavery came from the royalty, who gave serfs freedom through edicts and legislation in an attempt to broaden their tax base.
The absence of serfdom in some parts of medieval Europe raises several questions. Devroey thinks it is because slavery was not born out of economic structures in these areas, but was rather a societal practice. Heinrich Fichtenau points out that in Central Europe, there was not a labor market strong enough for slavery to become a necessity.
In late Rome, the official attitude towards slavery was ambivalent. According to Justinian's legal code, slavery was defined as "an institution according to the law of nations whereby one person falls under the property rights of another, contrary to nature." 
Justifications for slavery throughout the medieval period were dominated by the perception of religious difference. Slaves were often outsiders taken in war. As such, Hebrew and Islamic thinking both conceived of the slave as an "enemy within."  In the Christian tradition, pagans and heretics were similarly considered enemies of the faith who could be justly enslaved. In theory, slaves who converted could embark on the path to freedom, but practices were inconsistent: masters were not obliged to manumit them and the practice of baptising slaves was often discouraged. The enslavement of co-religionists was discouraged, if not forbidden, for Christians, Jews, and Muslims alike. Consequently, northern European pagans and black Africans were a target for all three religious groups. Ethnic and religious difference were conflated in the justification of slavery.
A major Christian justification for the use of slavery, especially against those with dark skin, was the Curse of Ham. The Curse of Ham refers to a biblical parable (Gen. 9:20–27) in which Ham, the son of Noah, sins by seeing his father inebriated and naked, although scholars differ on the exact nature of Ham's transgression. Noah then curses Ham's offspring, Canaan, with being a "servant of servants unto his brethren." Although race or skin color is not mentioned, many Jewish, Christian and Muslim scholars began to interpret the passage as a curse of both slavery and black skin, in an attempt to justify the enslavement of people of color, specifically those of African descent. In the medieval period, however, it was also used by some Christians as a justification for serfdom. Muslim sources in the 7th century allude to the Curse of Ham gaining relevance as a justifying myth for the Islamic world's longstanding enslavement of Africans.
The apparent discrepancy between the notion of human liberty founded in natural law and the recognition of slavery by canon law was resolved by a legal "compromise": enslavement was allowable given a just cause, which could then be defined by papal authority. The state of slavery was thought to be closely tied to original sin. Towards the middle of the 15th century, the Catholic Church, in particular the Papacy, took an active role in offering justifications for the enslavement of Saracens, pagans, Africans, and other "infidels." In 1452, a papal bull entitled Dum Diversas authorized King Afonso V of Portugal to enslave any "Saracens" or "pagans" he encountered. The Pope, Pope Nicholas V, recognized King Alfonso's military action as legitimate in the form of the papal bull, and declared the
full and free power, through the Apostolic authority by this edict, to invade, conquer, fight, subjugate the Saracens and pagans, and other infidels and other enemies of Christ, and...to lead their persons in perpetual servitude…
In a follow up bull, released in 1455 and entitled Romanus Pontifex, Pope Nicholas V reiterated his support for the enslavement of infidels in the context of Portugal's monopoly on North African trade routes.
Historians such as Timothy Rayborn have contended that religious justifications served to mask the economic necessities underlying the institution of slavery.
Whatever currency was in use [in Ireland in antiquity], it was not coin — as in other pre-coin economies, there was a system of conventional valuations in which female slaves, for example, were important units.
For the slave raiders, slaves were a valuable currency. You could sell them to buy wine and other luxury goods. There was always a market for them. There was always an unending supply of them, if only you were stronger than your neighbour. [...] For the Irish, slave-raiding was a lucrative extension to the cattle-raiding.